a) Filed of the Invention
The present invention relates to a back-driven-type autofocus (hereafter referred to as AF) camera in which a body for holding a film is adapted to be driven to an in-focus position on the basis of an amount of offset with respect to a focal plane on which an image of a subject is to be formed.
b) Description of Related Art
Techniques which have hitherto been disclosed as the focusing driving mechanism for a camera using a film include three systems, i.e., a system in which an image plane portion (aperture frame) of a camera body is moved in the direction of the optical axis, a system in which two mirrors provided between a photographic lens and a film-plane receiving portion are moved, and a system in which the photographic lens is moved in the direction of the optical axis.
The first system is disclosed in Japanese Utility Model Registration No. 294734 which was formerly put to commercial use. In this system since only the film-plane receiving portion is moved, it is necessary to provide a loop to the film between the film-plane receiving portion and two spool shafts holding the film, and it is necessary to provide a large interval between image planes so as to prevent the overlapping of the photographed image planes. Partly due to this reason, this system is not used for cameras in which winding is controlled by perforations in the film as in 35-mm film cameras which are presently used most widely, since the interval between the image planes is defined at a small dimension. In addition, in view of the recent tendency of the photographic lenses toward a larger aperture, it is necessary to increase the vertical accuracy of the film receiving portion with respect to the optical axis of the photographic lens, and it is difficult for this first system to meet this requirement during the focusing driving.
The second system is disclosed in Japanese Utility Model Examined Publication No. 16339/1990. In this system, the mirrors disposed between the photographic lens and the image-forming plane are moved to effect focus adjustment. Since two mirrors are required, this system must overcome the noise factor that the deterioration of an image on the film surface becomes large in proportion to the parallelism of the two mirrors after being built in, and meet the requirements of parallelism between the lens mounting surface and the film surface (parallelism of the camera body) which is generally required for cameras, parallelism between the two mirrors of a mirror assembly, and parallelism between the mirror assembly and the body. As a result, the required accuracy is high as compared with the techniques used in ordinary cameras, which results in increased cost, so that this system has not been put to commercial use.
The third system is one in which the photographic lens is moved in the direction of the optical axis, and has been widely disseminated as the focusing driving system of cameras because of the above-noted deficiencies of the other first and second systems and due to economic effects and the like. Also, this system can be further subclassified into the following types of system: a system in which the photographic lens at a distal end of a bellows is moved forward by a rack and a pinion, and which is used in press cameras; a system in which a lens barrel is moved forward by means of a lever interlocked with a rotating cam, and which is used in twin lens reflex cameras; and a system in which the lens is moved forward by rotating a helicoid of a photographic lens barrel, and which is presently used most widely.
Here, a description will be given hereafter of the progress made in the development of AF single-lens reflex cameras of the type in which the helicoid of the photographic lens is rotated. Namely, because of the above-described technical background, in single-lens reflex cameras of a non-AF lens interchangeable type, control concerning the diaphragm has been mainly effected by means of a mechanical signal pin at an interface between the photographic lens and the camera body with a lens mount interposed therebetween. With first-generation AF single-lens reflex cameras, a development target was placed on the expansion of the lens system which is principally based on conventional non-AF lenses, and an AF lens was provided as a part of the line of interchangeable photographic lenses. Namely, this is a combination of an AF photographic lens which incorporates a power supply and a motor for focusing driving and in which a connector terminal for communication with an arithmetic circuit on the camera body side including a rangefinding element is added to the lens mount, and a an AF camera body which is equipped with the rangefinding element and the arithmetic circuit and in which connector contacts for communication with the lens are added to the body mount. If the non-AF lens is mounted on the above-described AF camera, a focus aid function in which the camera-side rangefinding function works in manual focusing could be achieved. However, the aforementioned first-generation AF single-lens reflex cameras did not find widespread use since the AF lenses were large and heavy.
With respect to the second- and third-generation AF single-lens reflex cameras which were developed next, a series of AF systems have been developed in which a power supply and a focusing driving mother are incorporated on the camera side, a ROM with data of the Lens written therein or a CPU is mounted on the photographic lens so as to communicate information with a camera-side CPU, and a lens-driving coupler for focusing is provided as an interface for a mount. These AF single-lens reflex cameras contributed to the expansion of photographing opportunities as fully automatic cameras due in part to the automation of other elements.
On the other hand, however, the non-AF photographic lenses to date can demonstrate only the function of the above-described focus aid even if they can be mounted on the second -and third-generation AF single-lens reflex cameras. In realizing the AF operation, in order to speedily effect the automatic focusing or automate a large number of functions, information on interchangeable lenses is formed as ROM data and a CPU is mounted, so as to communicate information with the camera body-side CPU. In the above-described second- and third-generation AF single-lens reflex cameras, a coupler, an information-communication connector terminal, and the like are added as an interface function of the mount, thereby developing the lens-mounting mount as a totally new system. It can be said that this is a result of the fact that priority is placed on a new AF system rather than on the interchangeability of new and old lenses (interchangeable lenses exclusively used for AF, and non-AF interchangeable lenses).
In addition, to further reduce the time from rangefinding till focusing in the above-described lens- interchangeable-type AF single-lens reflex cameras, it is necessary to reduce the mass of the driving elements including lens elements to be driven for focusing among the elements making up the photographic lens, and to reduce the coefficient of friction of the driving system. For this purpose, various measures are adopted such as the use of an aspherical lens for a part of the lens assembly to reduce the total number of lens elements used, the use of a plastic molding for a lens barrel to reduce the mass of members for focusing driving, and the provision of a substantial clearance to the helicoid to reduce the coefficient of friction. However, if the helicoid is provided with a clearance, an amount of positional offset of lens elements from the optical axis and a misalignment of some lenses or the entire group of lenses from the optical axis occur due to the difference in the attitude of the camera. In particular, if the aspherical lenses are used, peripheral blurring and the deterioration of image performance such as MTF become noticeable due to the misalignment of the optical axis.
The performance of the photographic lens including the lens barrel has been developed by precision technology over a long period of years. This being the case, however, the provision of a clearance to the helicoid to reduce the focusing time resulted in harmful defects, i.e., the deterioration of image performance on the film surface and the offset of the focal plane. If the concept of the optical design for obtaining high-quality image performance is viewed in the light of the AF camera technology as a whole, the results have not been sufficiently satisfactory.
Moreover, the above-described second- and third-generation lens-interchangeable-type AF single-lens reflex cameras had problems is a camera system before the above-described techniques. That is, although it depends on the way the interchangeable lenses are chosen, interchangeable lens groups are generally more expensive than the cameras. If an AF single-lens reflex camera is to be used, non-AF interchangeable lens groups cannot be used at all if the focus aid is deteriorated. The camera users are denied the non-AF interchangeable lens systems which they purchased before, so that the users are compelled to replace the non-AF lens systems newly with AF interchangeable lens systems, or separately restructure the AF interchangeable lens systems, thereby imposing substantial economic sacrifices upon the users. If the progress of the development of camera technologies to date is viewed, the interchangeable function of lenses remains practically unchanged, and the unreasonableness concerning the above-described lens systems is caused from the fact that the lens systems are changed on the basis of a viewpoint centered around the camera body by being bewildered by the phrase of the interchangeable lens.
Ten-odd years have elapsed since the marketing of the above-described AF single-lens reflex camera systems. These AF single-lens reflex camera systems have received appraisal on their own merits and found widespread use, but the above-described problems remain unresolved. The technologies concerning the helicoid mechanism of the lens barrel have been developed over a long period to operate the helicoid mechanism smoothly without play and increase the economic efficiency by placing the lenses on the optical axis in the upright position. Japanese Patent Unexamined Publication No. 130007/1982 discloses a method in which sliding friction is converted to rolling friction while using a precision helicoid so as to reduce the coefficient of friction for focusing driving. With this method, however, since the surface hardness of the helicoid is increased, and steel balls are placed on the entire periphery, the cost becomes very high, and the total cost becomes enormous to implement a totally AF interchangeable lens. Hence, this method has not been put to commercial use.
For this reason, the aforementioned non-AF lens-interchangeable-type single-lens reflex cameras coexist with the AF single-lens reflex cameras without being weeded out. When systems for single-lens reflex cameras are considered, it is thought that various cameras can be developed centering on interchangeable lens groups. Namely, it is natural to use interchangeable bodies.